Method for drilling directional wells

ABSTRACT

A method for detecting and sensing boundaries between strata in a formation during directional drilling so that the drilling operation can be adjusted to maintain the drillstring within a selected stratum is presented. The method comprises the initial drilling of an offset well from which resistivity of the formation with depth is determined. This resistivity information is then modeled to provide a modeled log indicative of the response of a resistivity tool within a selected stratum in a substantially horizontal direction. A directional (e.g., horizontal) well is thereafter drilled wherein resistivity is logged in real time and compared to that of the modeled horizontal resistivity to determine the location of the drill string and thereby the borehole in the substantially horizontal stratum. From this, the direction of drilling can be corrected or adjusted so that the borehole is maintained within the desired stratum.

BACKGROUND OF THE INVENTION

This invention relates generally to borehole formation evaluationinstrumentation and methods of using such instrumentation in thedrilling of directional wells. More particularly, this invention relatesto a method for measuring the position of a drillstring while drilling ahorizontal borehole and maintaining the drillstring within desiredboundaries using electromagnetic propagation based earth formationevaluation tools.

Borehole formation evaluation tools are known which measure phase and/oramplitude of electromagnetic waves to determine an electrical property(such as resistivity or permitivity) of a section of a borehole.Typically, the existing tools used for this application are composed ofone or more transmitting antennas spaced from one or more pairs ofreceiving antennas. An electromagnetic wave is propagated from thetransmitting antenna into the formation surrounding the borehole and isdetected as it passes by the two receiving antennas. In a resistivitymeasuring tool, magnetic dipoles are employed which operate in the mfand lower hf spectrum. In contrast, permitivity tools utilize electricdipoles in the VHF or UHF and microwave ranges.

In a known resistivity sensor of the type hereinabove discussed which isused by Teleco Oilfield Services, Inc., assignee of the presentapplication, the resistivity sensor measures both phase difference andamplitude ratio to provide two resistivities with different depths ofinvestigation. A signal received in a first (far) receiving antenna isshifted in phase and its amplitude will be less than the signal receivedin a second (near) receiving antenna. Resistivities are then derivedfrom both the phase difference (R_(pd)) and the amplitude ratio (R_(ar))of the received signals. The differential measurement is primarilyresponsive to the formation opposite the receiving antennas and is lesssensitive to the borehole and/or variations in the transmitted signal asin prior art sensing devices. An example of a formation evaluationinstrument of this type is described in FIGS. 1 and 2 of U.S. Pat. No.5,001,675 which is assigned to the assignee hereof and fullyincorporated herein by reference. The formation evaluation tool acquiresthe resistivity data in real time and then transmits this information tothe drilling operator using any suitable measurement-while-drillingtransmission technique such as mud pulse telemetry (see U.S. Pat. Nos.3,982,431, 4,013,945 and 4,021,774) or the information is storeddownhole for review after retrieval of the tool.

In drilling a horizontal well, the goal is to drill the well in such away that the well stays within the pay zone (i.e., a selected bed orstratum) for as long as possible in order to achieve a higher recoveryrate. Before drilling a horizontal well, a course of drilling is plannedbased on knowledge about the pay zone and the boundaries between the payzone and its neighboring beds. Because of the uncertainties in theknowledge about the bed boundaries and the errors in directionaldrilling, the well being drilled often can not stay in the pay zone asdesired. It is, therefore, very important to know the relative positionof the drill bit and the well with respect to the bed boundary in realtime so that when the well is very close to the bed boundary, properaction is taken, ensuring that the well will stay within the pay zone.

SUMMARY OF THE INVENTION

The above-discussed and other drawbacks and deficiencies of the priorart are overcome or alleviated by the method for drilling directionalwells (e.g., horizontal wells) of the present invention. In accordancewith the present invention, this drilling method comprises drilling asubstantially vertical well (generally referred to as an offset well)and generating a first log of resistivity as a function of depth or, aswill be discussed later, an assumed log is generated from knowledgeother than a vertical well. Using any one of a number of known dippingbed modeling techniques, this first log is then modeled to provide asecond log which is indicative of resistivity of a selected bed (orstratum) along a horizontal or substantially horizontal direction. Adirectional well (a second well or borehole) is then drilled near theoffset well. The directional well is initially vertical and turnssubstantially horizontal in the substantially horizontal (previouslyselected) bed. A measurement-while-drilling (MWD) electromagneticpropagation tool is employed during the directional drilling to providea third log (i.e., a horizontal log) while drilling (e.g., in realtime). The third log is compared to the second log (i.e. the modeledhorizontal log) so that the location of the drillstring (and thereforethe horizontal well) can be determined and the direction of drillingcorrected or adjusted accordingly so that the drillstring is maintainedwithin the desired stratum.

Bed boundaries (i.e., where adjacent beds meet) cause three distinctfeatures (i.e., separation, dips and horns) in the horizontal logs. Inaccordance with the above-described method of this invention, thesefeatures can be used to detect the presence of the bed boundaries indrilling horizontal or nearly horizontal wells (i.e., the directionalwell).

In another embodiment of this invention, no initial vertical well isdrilled. Instead, assumptions are made about the resistivities of thevertical strata from other information such as geological knowledge ofthe area or siesmic measurements. These assumed resistivities are usedin the aforementioned dipping bed model to generate the modeled log ofthe substantially horizontal stratum. Thereafter, as in the firstembodiment, the real time resistivity log of the directional well iscompared to the modeled log to correct or adjust the drilling operationso as to maintain the drillstring within a desired substantiallyhorizontal stratum.

Modeling can also be carried out on site with the position of theresistivity tool relative to the bed boundary as an input to the model.The tool's position relative to the bed boundaries is varied in themodel calculations until the real time resistivity log is reproduced. Byso doing, the tool's Position, and therefore the drill bit and wellrelative to the bed, is determined.

The above-discussed and other features and advantages of the presentinvention will be appreciated and understood by those skilled in the artfrom the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, wherein like elements are numbered alikein the several FIGURES:

FIG. 1 is front elevation view, partly in cross section, of a portion ofan electromagnetic propagation resistivity tool in a substantiallyvertical borehole;

FIG. 2 is a front elevation view, partly in cross section, of theelectromagnetic propagation resistivity tool in a substantiallyhorizontal portion of a directional borehole;

FIG. 2A is a schematic diagram of an electromagnetic propagationresistivity tool shown passing through several bed boundaries at aselected dipping angle;

FIG. 3 is a schematic diagram of the electromagnetic propagationresistivity tool in the directional well of FIG. 2A;

FIGS. 4A-4E are charts showing the phase difference resistivity andamplitude ratio resistivity of the electromagnetic propagationresistivity tool in the directional borehole of FIG. 2A, FIG. 4A showsresistivity as the tool travels into and out of a 50 ohm-meter bed in avertical well (0 degree dipping angle) FIG. 4B shows resistivity as thetool travels into and out of the 50 ohm-meter bed at a 45 degree dippingangle, FIG. 4C shows resistivity as the tool travels into and out of the50 ohm-meter bed at a 60 degree dipping angle, FIG. 4D shows resistivityas the tool travels into and out of the 50 ohm-meter bed at a 75 degreedipping angle, and FIG. 4E shows resistivity as the tool travels intoand out of the 50 ohm-meter bed at a 90 degree dipping angle (horizontalwell);

FIG. 5 is a modeled response of phase difference resistivity andamplitude ratio resistivity of the electromagnetic propagationresistivity tool of FIG. 2A in a 200 ohm-meter bed at a 90 degree dipangle;

FIG. 6 is a log chart of a first experimental directional well inaccordance with the present invention;

FIG. 7 is a log chart of a second experimental directional well inaccordance with the present invention; and

FIG. 8 is a chart of a model of the log of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Naturally formed geological bed boundaries are not necessarilyhorizontal and may not be perfect planes. During directional drilling(i.e., horizontal drilling), a predetermined course of drilling cannotensure that the well being drilled stays in the producing bed. Thus, animportant concern in horizontal drilling is early detection of the bedboundary so that the well can be maintained in the hydrocarbon producingbed.

Referring to FIG. 1, a substantially vertical well 10 (i.e., borehole ordipping bed) is drilled near a desired well location in accordance withwell known methods. Well 10 is a vertical offset well from whichformation resistivity information is initially collected (i.e., a log).An electromagnetic propagation resistivity tool 12 (generallyincorporated into a drill string 14 for measurement-while-drilling) isshown disposed in well 10. Tool 12 is typically two MHz. Well 10 extendsacross several formations identified at 15, 16, 17 and 18. Well 10 hasdrilling fluid 19 flowing therein. Tool 12 includes a transmitterantenna 20, a near receiver antenna 22 and a far receiver antenna 24. Inthis example, the distances between transmitter antenna 20 and near andfar receiver antennas 22,24 are 27.5 and 34.5 inches, respectively.Antennas 20, 22 and 24 are disposed on a drill collar which houses theelectronics for transmitting a electromagnetic wave by antenna 20 to bedetected by antennas 22 and 24. Tool 12 is known in the art anddescribed in FIG. 1 of aformentioned U.S. Pat. No. 5,001,675.

In accordance with a first embodiment of the method of this invention,resistivity as a function of depth is logged by tool 12 so that theresistivities in the formations 15-18 and the bed boundaries whichseparate them can be determined. This resistivity measurement is thenused in dipping bed modeling to approximate the response of tool 12during horizontal drilling. As will be discussed with reference to FIG.2, during actual drilling of horizontal or near horizontal wells using adrillstring which incorporates tool 12, resistivity readings from tool12 are compared to the model and used to detect bed boundaries inhorizontal and near horizontal wells. Based on these comparisons,adjustments to the directional drilling operations can be made tomaintain the drillstring within a desired pay zone or stratum.

The model approximates the antennas 20, 22, 24 as dipoles (the steeldrill string 14 and borehole effects are not included). The dipoleapproximation provides a good approximation except when the tool iswithin a few inches of a bed boundary (e.g., between strata 17 and 18)and the dipping angle is large (as described hereinafter). There aredifferent, well known methods which can be used to carry out dipping bedmodel calculations. In the following example, the model described in anarticle entitled "Computation of Induction Logs in Multiple-LayerDipping Formation", IEEE Trans. on Geoscience and Remote Sensing, Vol.27, No. 3 (1989), p. 259, by Ming Huang and Liang C. Shen is used, theentire contents of which is incorporated herein by reference.

Referring now to FIG. 2, a second well or borehole 26 (i.e., adirectional well) is drilled which is initially vertical and isthereafter directed generally horizontally into a pay zone (i.e.,hydrocarbon producing bed 17) selected from the the offset well 10resistivity log or otherwise. By comparing the modeled tool response tothe real time tool response during horizontal drilling, it can bedetermined approximately where in well 26 the tool 12' is, and therebythe location of the borehole. The direction of drilling is shown byarrow 28. Based on this comparison, the drilling operator can adjustand/or correct the directional drilling operations to maintain thedrilling in a desired stratum. 2A is a diagrammatic view of thedirection well at any angle θ with FIG. 2 being the special case where θequals 90 degrees.

EXAMPLES

In this example the thickness of the hydrocarbon producing bed 17 is 60feet. Bed 17 and the shoulder resistivities are 50 and 2 ohm-meters,respectively. A schematic plot of the formation geometry and the tool isshown in FIG. 3. The true vertical depth 32 is defined as the distancebetween the upper bed boundary 34 and the midpoint between the tworeceiver antennas 36 (the tool measure point). A negative true verticaldepth means that the midpoint is in the upper shoulder bed. In thefollowing, the distance between the tool and the bed boundary is themagnitude of the true vertical depth.

Referring to FIGS. 4A-4E, apparent resistivities are plotted asfunctions of vertical depth for 0, 45, 60, 75, and 90 degree dippingangles, respectively. Dipping angle 38 (FIG. 3) is defined as the anglebetween the borehole and the perpendicular to the bedding plane. On avertical depth plot, the effect of dipping is negligible for dippingangles from 0 degrees up to about 45 degrees (FIG. 4A and 4B). At about60 degrees (FIG. 4C), horns 40 in apparent resistivities (i.e., phasedifference resistivity and amplitude ratio resistivity) appear aroundthe bed boundaries. Horns 40 are produced by the interference betweenthe electromagnetic wave generated by the transmitter antenna 20 as wellas the bulk eddy currents and the electromagnetic wave generated by thesurface charges on the bed boundary caused by the discontinuity. A solidline 42 represents phase difference resistivity and a dashed line 44represent amplitude ratio resistivity. As the dipping angle 38increases, the horns 40 become more pronounced (e.g., at 75 degrees,FIG. 4D). Near 90 degrees (FIG. 4E), the values of horns 40 are offscale and the width of the phase resistivity horn is about 1.5 feet.Near 90 degrees, the response does not change very much with the dippingangle. For example, the response at 85 degrees is almost identical tothat of 90 degrees as functions of true vertical depth. In horizontalwell 26 inside a relatively resistive bed, the phase differenceresistivity readings drop when the tool is about 4 feet away from theconductive boundary and then rise drastically as the tool approaches thebed boundary. Phase and amplitude resistivities separate when the toolis about 10 feet away from the boundary. The separation becomessignificant when the distance between the tool and the boundary is 6feet.

In this particular example, three features can be used to detect aconductive bed boundary is a horizontal well; resistivity horns 40;resistivity drops; and phase and amplitude resistivity separation. Thephase resistivity horns 40 are detectable when the tool is about 1 footaway from the bed boundary. The resistivity drops can identify the bedboundary presence when tools are about 4 feet away. Bed boundaries causea significant phase and amplitude resistivity separation when tools areup to 6 feet away.

The phase and amplitude resistivity separation feature are preferred toidentify the bed boundary since it provides the earliest detection. Whenthe tool 12 is 6.5 feet away from the bed boundary, the separationbetween the phase and amplitude resistivities is about 20 percent of thetrue bed resistivity. If the phase and the amplitude resistivityseparation caused by other factors (e.g., invasion, tool eccentering ordielectric effects) is less than 20 percent, the bed boundary can bedetected when the boundary is 6.5 feet away. At 87 or 93 degree dippingangle, 6.5 foot vertical depth corresponds to 130 feet in measured holedepth. If 30 percent separation criteria is used, the bed boundary canbe detected when the tool is 5 feet away from the boundary. In practice,however, many factors may cause phase and amplitude resistivityseparations. Amplitude resistivity readings of tool 12 may not bereliable at very high resistivities. Therefore, separation of phase andamplitude curves can provide early bed boundary detection in a limitednumber of situations.

When amplitude resistivity readings are not reliable, the phaseresistivity drop feature is preferred for the detection of conductivebed boundaries. The phase resistivity is 8 percent lower than the bedresistivity if the tool is 4 feet away from the bed boundary. If 20percent drop criteria is used, the bed boundary can be identified whentool 12 is 2.5 feet away. At 87 or 93 degrees. 2.5 feet in verticaldepth corresponds to 50 feet in measured depth. If the drill bit is 20feet ahead of the tool, the bed boundary can be detected when the drillbit is 1 foot away from the bed boundary.

The horns in resistivity readings provide the most unambiguous signatureof the bed boundary presence since no other known physical effect canproduce a horn of this magnitude in apparent resistivity readings.However, the tool has to be about 1.5 feet or closer to the bed boundaryto detect these horns. In most cases, when a horn 40 shows up in theresistivity log, the drill bit has already crossed the boundary. Insituations where the penalty for the bit's crossing the bed boundary issmall or none, horns 40 can be used as the bed boundary detector. In anycase, horns 40 on resistivity logs signify with certainty the presenceof a bed boundary.

In a second example similar to the first example, the bed thickness andthe shoulder resistivity are same as those of the first example. The bedresistivity is 200 ohm-meters. In FIG. 5, phase and amplituderesistivities 42',44' are plotted as functions of true vertical depth.The dipping angle is 90 degrees. The three features are similar to thosein the first example (FIGS. 4A-D) with only quantitative differences.The Phase and amplitude resistivity separation is 20 percent of the bedresistivity when the tool is 12 feet from the boundary. At 10 feet theseparation is 30 percent. The phase resistivity drop is 20 percent at 4feet. The horn width is about the same as that in the first example.

Therefore horns, resistivity drops, and phase and amplitude resistivityseparations can be used to detect the presence of a bed boundary. Itwill be appreciated that this method of maintaining a directionalborehole in a bed (i.e., detection of bed boundaries) can be employedwithout the offset well and, thus without the vertical resistivity logsfor comparison. The horns provide the most reliable detection, but thetool has to be about 1.5 feet from the boundary. The phase and amplituderesistivity separation feature can detect the boundary from a relativelylarge distance. This feature can not be used when other factors causelarge phase and amplitude resistivity separations or when the amplituderesistivity readings are not reliable.

FIELD TEST

Referring to FIG. 6, in a first field test, a wireline induction log 46of a vertical well (in accordance with a first step of the method ofthis invention and corresponding to the vertical drilling shown inFIG. 1) indicates that there is a 60 foot thick gas producing sand bedlocated at about 1700 feet below the sea level. The upper shoulder isshale whose resistivity is about 2 ohm-meters. The lower shoulder is awater and sand stratum with a resistivity of 0.6 ohm-meters. Theinduction resistivity in the bed varies significantly. The average isabout 50 ohm-meters, without shoulder bed effect correction. The bed isnot completely horizontal, but the dip is at most a few degrees.

In order to avoid water coning, it is desired that the horizontal wellsshould stay in the top third of the producing bed. In drilling thehorizontal portions of the wells, 2 MHz electromagnetic resistivity MWDtools of the type described with reference to U.S. Pat. No. 5,001,675are used.

Resistivity log 46 shows phase difference resistivity (RPD), amplituderatio resistivity (RAR), true vertical depth (TVD), natural gamma ray,and inclination are plotted as a function of measured hole depth. Twohorns are present on the phase resistivity curve 48. From the verticaldepth reading, one can see that both horns indicate the upper bedboundary crossing at 40. The first horn 52 signifies the tool enteringinto the pay bed from the upper shoulder. The second horn 54 shows thetool leaving the bed and entering the upper shoulder. Unfortunately,since 200 ohm-meters is above the range of the amplitude ratioresistivity measurement, the phase and amplitude resistivity separationfeature can not be seen. The phase resistivity drop feature is present.Since the phase resistivity reading in the bed fluctuates somewhat, onemust be cautious when using the phase resistivity drop feature to detectthe bed boundary in situations like this.

Referring to FIG. 7, a log 56 from a second field test is shown. As inthe previous well log (FIG. 6), the tool entered the bed from the uppershoulder and again exited the bed into the upper shoulder. Two horns 58appear when the tool crosses the upper bed boundary. The double hornsare likely caused by another layer of sand in the upper shoulder.

In accordance with a second step of this-invention, the log 56 in FIG. 7is modeled in a dipping bed model (such as the aforementioned model byHuang and Shen). FIG. 8 shows the model response obtained with a onefoot sand layer located 3 feet above the 60 foot sand bed. The wellenters the pay sand at 87 degrees and leaves the pay sand at 93 degrees.The hole depth offset is arbitrarily chosen. The well path as a functionof hole depth is also shown. As mentioned, the model response in FIG. 8reproduces, at least qualitatively, the log 56 in FIG. 7. The amplituderesistivity under the first phase resistivity horn, according to themodel, is measurable, but it is not around 20 ohm-meters which is shownin log 56.

Log 56 (FIG. 7) is essentially symmetrical about an midpoint of 3425feet measured depth. This phenomenon is due to the fact that this depthcorresponds to the deepest stratigraphic penetration into the producingsand. At measured depths below 3435, the well has turned upward (aninclination greater than 90 degrees) and is drilling stratigraphicallyupwards out of the top of the producing sand. The section below 3435 iscompressed with respect to the section above this point due to a greaterapparent dip angle between the borehole and the formation. Similarsymmetry can be seen in log 46 (FIG. 6), which reaches its deepeststratigraphic penetration at roughly 3600 feet measured depth. Also notethe similarity between these actual logs 46, 56 and the modeled logresponse of FIG. 8.

The model response study shows that the horns, the resistivity drops,and the phase and amplitude resistivity separations in horizontal welllogs are due to the bed boundary presence. The three features(resistivity horns, resistivity drops and phase and amplituderesistivity separations) can be used to detect the presence of bedboundaries. The resistivity horns and the phase resistivity drop featureare present in both horizontal well logs 46, 56.

As a bed boundary is detected, this information is passed on to thedirectional drilling operator who will adjust the drilling parameters toalter the drilling direction and thereby maintain the drillstring withinthe desired horizontal stratum.

In a second embodiment of this invention, no vertical well is required.Instead, assumptions are made about the resistivities of the verticalstrata from other information such as geological knowledge of the areaor seismic measurements. These assumed resistivities are used in theaforementioned dipping bed model to generate the modeled log of thesubstantially horizontal stratum. Thereafter, as in the firstembodiment, the electromagnetic propagation resistivity tool is used inthe directional well to obtain a real time resistivity log of thesubstantially horizontal stratum. That log is then compared to themodeled log with the comparison being used to adjust or correct thedrilling operations to maintain the drillstring within the desiredstratum.

In a third embodiment of this invention, either the actual dipping bedmodel or a library of the output resistivity logs of the dipping bedmodel for varying conditions is employed in real time as the well isactually being drilled. The term "library" means a group of previouslygenerated dipping bed model log outputs for various expected downholeconditions (similar to a "look-up" table). As the real time resistivitylog is the real time generated model resistivity logs or with thelibrary of modeled resistivity logs. Input parameters to the model or tothe library are then modified until the output agrees or matches withthe real time resistivity log. At that time the input parameters areassumed to be correct. The input parameters consist of a selectedcombination of the bed resistivities, the bed thicknesses and thedipping angle. The input parameters from the matched modeled log can beused to assist the drilling operator in maintaining the drillstringwithin the selected stratum.

While the the preferred embodiments have been shown and described,various modifications an substitutions may be made thereto withoutdeparting from the spirit and scope of the invention. Accordingly, it isto be understood that the present invention has been described by way ofillustrations and not limitations.

What is claimed is:
 1. A method of locating a substantially horizontalbed of interest in a formation and maintaining a drillstring thereinduring the drilling operation, said drillstring including ameasurement-while-drilling (MWD) electromagnetic propagation resistivitysensor, comprising the steps of:drilling a substantially vertical offsetwell in a formation having at least one selected substantiallyhorizontal bed therein; measuring resistivity in the formation at theoffset well to provide a first resistivity log as a function of depth;modeling the substantially horizontal bed to provide a modeledresistivity log indicative of the resistivity taken along thesubstantially horizontal bed, said modeling being based on said firstresistivity log; drilling a directional well in said formation near saidoffset well, a portion of said directional well being disposed in saidsubstantially horizontal bed; measuring resistivity in said directionalwell using the MWD electromagnetic propagation resistivity sensor toprovide a second log of resistivity taken substantially horizontally;comparing said second log to said modeled log to determine the locationof said directional well; and adjusting the directional drillingoperation so as to maintain said drillstring within said substantiallyhorizontal bed during the drilling of said directional well in responseto said comparing step.
 2. The method of claim 1 wherein said second logand said modeled log comprises:at least one resistivity horn beingindicative of a boundary between said bed and an adjacent bed.
 3. Themethod of claim 2 wherein said horn is a phase difference resistivityhorn.
 4. The method of claim 2 wherein said horn is an amplitude ratioresistivity horn.
 5. The method of claim 1 wherein said second log andsaid modeled log are phase difference resistivity logs.
 6. The method ofclaim 1 wherein said second log and said modeled resistivity log areamplitude ratio resistivity logs.
 7. The method of claim 1 wherein saidadjusting step further includes:maintaining said drillstring within theupper portion of said bed during the drilling of said directional wellin response to said comparing.
 8. A method of locating a substantiallyhorizontal bed of interest in a formation and maintaining a drillstringtherein during the drilling operation, said drillstring including ameasurement-while-drilling (MWD) electromagnetic propagation resistivitysensor, comprising the steps of:drilling a substantially vertical offsetwell in a formation having at least one selected substantiallyhorizontal bed therein; measuring resistivity in the formation at thewell to provide a first resistivity log as a function of depth; modelingthe substantially horizontal bed to provide a modeled resistivity logindicative of the resistivity taken along the substantially horizontalbed, said modeling being based on said first resistivity log; drilling asubstantially horizontal well within said substantially horizontal bed;measuring resistivity in said horizontal well using the MWDelectromagnetic propagation resistivity sensor to provide a second logof resistivity taken substantially horizontally; comparing said secondlog to said modeled log to determine the location of said horizontalwell; and adjusting the horizontal drilling operation so as to maintainsaid drillstring within said substantially horizontal bed during thedrilling of said well in response to said comparing step.
 9. A method oflocating a substantially horizontal bed of interest in a formation andmaintaining a drillstring therein during the drilling operation, saiddrillstring including a measurement-while-drilling (MWD) electromagneticpropagation resistivity sensor, comprising the steps of:generating afirst resistivity log for a substantially horizontal bed using assumedresistivity values for a desired course of drilling in a preselecteddipping bed model to provide a modeled resistivity log indicative of theresistivity taken along the substantially horizontal bed in a formation;drilling a directional well in said formation, a portion drilling adirectional well in said formation, a portion of said directional wellbeing disposed in said substantially horizontal bed; measuringresistivity in said directional well using the MWD electromagneticpropagation resistivity sensor to provide a second log of resistivitytaken substantially horizontally; comparing said second log to saidmodeled first log to determine the location of said directional well;and adjusting the directional drilling operation so as to maintain saiddrillstring within said substantially horizontal bed during the drillingof said directional well in response to said comparing step.
 10. Themethod of claim 9 wherein said second log and said modeled logcomprises:at least one resistivity horn being indicative of a boundarybetween said bed and an adjacent bed.
 11. The method of claim 10 whereinsaid horn is a phase difference resistivity horn.
 12. The method ofclaim 10 wherein said horn is an amplitude ratio resistivity horn. 13.The method of claim 9 wherein said second log and said modeled log arephase difference resistivity logs.
 14. The method of claim 9 whereinsaid second log and said modeled resistivity log are amplitude ratioresistivity logs.
 15. The method of claim 9 wherein said adjusting stepfurther includes:maintaining said drillstring within the upper portionof said bed during the drilling of said directional well in response tosaid comparing.
 16. A method of locating a substantially horizontal bedof interest in a formation and maintaining a drillstring therein duringthe drilling operation, said drillstring including ameasurement-while-drilling (MWD) electromagnetic propagation resistivitysensor, comprising the steps of:generating a group of dipping bed modellogs for various expected downhole conditions; drilling a directionalwell in a formation, a portion of said directional well being disposedin a substantially horizontal bed; measuring resistivity in saiddirectional well using the MWD electromagnetic propagation resistivitysensor to provide a real time log of resistivity taken substantiallyhorizontally; comparing said real time log to at least one of the modellogs in said group of dipping bed model logs until one of said modellogs substantially matches said real time log defining a matched modellog; using input parameters from said matched model log to determine thelocation of said directional well; and adjusting the directionaldrilling operation so as to maintain said drillstring within saidsubstantially horizontal bed during the drilling of said directionalwell in response to said comparing step.
 17. A method of locating asubstantially horizontal bed of interest in a formation and maintaininga drillstring therein during the drilling operation, said drillstringincluding a measurement-while-drilling (MWD) electromagnetic propagationresistivity sensor, comprising the steps of:drilling a directional wellin a formation, a portion of said directional well being disposed in asubstantially horizontal bed; measuring resistivity in said directionalwell using the MWD electromagnetic propagation resistivity sensor toprovide a real time log of resistivity taken substantially horizontally;generating a model resistivity log from a preselected dipping bed modelfor a substantially horizontal bed using input values selected so thatthe model resistivity log will substantially match the real time logdefining a matched model log; using input parameters from said matchedmodel log to determine the location of said directional well; andadjusting the directional drilling operation so as to maintain saiddrillstring within said substantially horizontal bed during the drillingof said directional well in response to said comparing step. .Iadd. 18.A method of locating a bed of interest in a formation and maintaining adrillstring therein during the drilling operation, said drillstringincluding a measurement-while-drilling (MWD) resistivity sensor,comprising the steps of:(1) modeling at least one substantiallyhorizontal bed to provide a modeled resistivity log indicative of theresistivity taken along the substantially horizontal bed, said modelingbeing based on information related to said substantially horizontal bed;(2) drilling a directional well, a portion of said directional wellintended to be disposed in said substantially horizontal bed; (3)measuring resistivity in said directional well using the MWD resistivitysensor to provide a measured log of resistivity; and (4) comparing themeasured log to said modeled log to determine the location or positionof said directional well with respect to said substantially horizontalbed. .Iaddend..Iadd.19. The method of claim 18 including the step of:adjusting the directional drilling operation so as to maintain saiddrillstring within said substantially horizontal bed during the drillingof said directional well in response to said comparing step..Iaddend..Iadd.20. The method of claim 18 wherein: said information isselected from at least one of (a) data from an offset well whichintersects with said horizontal bed, (b) seismic data, (c) assumptionsof the horizontal bed, and (d) prior geological knowledge of thehorizontal bed. .Iaddend..Iadd.21. The method of claim 18 including thesteps of:identifying at least one resistivity horn in said measured log;and using said resistivity horn as an indicator of a boundary betweensaid substantially horizontal bed and an adjacent bed..Iaddend..Iadd.22. The method of claim 21 wherein said comparing stepfurther includes: comparing said measured resistivity horn to saidmodeled resistivity log to determine the location or position of saiddirectional well. .Iaddend..Iadd.23. The method of claim 21 wherein saidhorn is a phase difference resistivity horn. .Iaddend..Iadd.24. Themethod of claim 21 wherein said horn is an amplitude ratio resistivityhorn. .Iaddend..Iadd.25. The method of claim 18 wherein said measuredlog and said modeled log are phase difference resistivity logs..Iaddend..Iadd.26. The method of claim 18 wherein said measured log andsaid modeled log are amplitude ratio resistivity logs..Iaddend..Iadd.27. The method of claim 18 wherein said measured log andsaid modeled log include both phase difference resistivity logs andamplitude ratio resistivity logs. .Iaddend..Iadd.28. The method of claim19 wherein said adjusting step further includes:maintaining saiddrillstring within the upper portion of said bed during the drilling ofsaid directional well. .Iaddend..Iadd.29. The method of claim 18 whereinsaid measured log includes resistivity from phase difference andresistivity from amplitude ratio and including the steps of: identifyingseparations between said phase difference and amplitude ratio in saidmeasured log; and using said separations as an indicator of a boundarybetween said substantially horizontal bed and an adjacent bed..Iaddend..Iadd.30. The method of claim 18 including the steps of:identifying resistivity drops in said measured log; and using saidresistivity drops as an indicator of a boundary between said bed and anadjacent bed. .Iaddend..Iadd.31. The method of claim 18 wherein: saidMWD resistivity sensor comprises an electromagnetic propagationresistivity tool. .Iaddend..Iadd.32. The method of claim 18 wherein:said measured log and said modeled log are derived from at least one ofphase and amplitude measurements of an electromagnetic propagationresistivity tool. .Iaddend..Iadd.33. The method of claim 18 wherein:saidmeasured log and said modeled log are derived from at least one of phaseand amplitude measurements of an electromagnetic propagation resistivitytool. .Iaddend..Iadd.34. A method of locating a bed of interest in aformation and maintaining a drillstring therein during the drillingoperation, said drillstring including a measurement-while-drilling (MWD)resistivity sensor, comprising the steps of:measuring resistivity in theformation at an offset well to provide a first resistivity log as afunction of depth, the offset well having at least one selectedsubstantially horizontal bed therein; modeling the substantiallyhorizontal bed to provide a modeled resistivity log indicative of theresistivity taken along the substantially horizontal bed, said modelingbeing based on said first resistivity log; drilling a directional wellin said formation near said offset well, a portion of said directionalwell intended to be disposed in said substantially horizontal bed;measuring resistivity in said directional well using the MWD resistivitysensor to provide a second log of resistivity taken substantiallyhorizontally; and comparing said second log to said modeled log todetermine the location or position of said directional well with respectto said substantially horizontal bed. .Iaddend..Iadd.35. The method ofclaim 34 including the step of: adjusting the directional drillingoperation so as to maintain said drillstring within said substantiallyhorizontal bed during the drilling of said directional well in responseto said comparing step. .Iaddend..Iadd.36. The method of claim 34wherein said adjusting step further includes:maintaining saiddrillstring within the upper portion of said bed during the drilling ofsaid horizontal well. .Iaddend..Iadd.37. A method of locating a bed ofinterest in a formation and maintaining a drillstring therein during thedrilling operation, said drillstring including ameasurement-while-drilling (MWD) resistivity sensor, comprising thesteps of:(1) modeling at least one substantially horizontal bed toprovide a modeled resistivity log indicative of the resistivity takenalong the substantially horizontal bed, said modeling being based oninformation related to said substantially horizontal bed; (2) drilling asubstantially horizontal well within said substantially horizontal bed,a portion of said horizontal well intended to be disposed in saidsubstantially horizontal bed; (3) measuring resistivity in saidhorizontal well using the MWD resistivity sensor to provide a measuredlog of resistivity taken substantially horizontally; and (4) comparingthe measured log to said modeled log to determine the location orposition of said horizontal well with respect to said substantiallyhorizontal bed. .Iaddend..Iadd.38. The method of claim 37 including thestep of: adjusting the directional drilling operation so as to maintainsaid drillstring within said substantially horizontal bed during thedrilling of said horizontal well in response to said comparing step..Iaddend..Iadd.39. The method of claim 37 wherein: said information isselected from at least one of (a) data from an offset well which bed,and (d) prior geological knowledge of the horizontal bed..Iaddend..Iadd.40. The method of claim 37 including the stepsof:identifying at least one resistivity horn in said measured log; andusing said resistivity horn as an indicator of a boundary between saidsubstantially horizontal bed and an adjacent bed. .Iaddend..Iadd.41. Themethod of claim 40 wherein said comparing step further includes:comparing said measured resistivity horn to said modeled resistivity logto determine the location or position of said directional well..Iaddend..Iadd.42. The method of claim 40 wherein said horn is a phasedifference resistivity horn. .Iaddend..Iadd.43. The method of claim 40wherein said horn is an amplitude ratio resistivity horn..Iaddend..Iadd.44. The method of claim 37 wherein said measured log andsaid modeled log are phase difference resistivity logs..Iaddend..Iadd.45. The method of claim 37 wherein said measured log andsaid modeled log are amplitude ratio resistivity logs..Iaddend..Iadd.46. The method of claim 37 wherein said measured log andsaid modeled log include both phase difference resistivity logs andamplitude ratio resistivity logs. .Iaddend..Iadd.47. The method of claim37 wherein said measured log includes resistivity from phase differenceand resistivity from amplitude ratio and including the stepsof:identifying separations between said phase difference and amplituderatio in said measured log; and using said separations as an indicatorof a boundary between said bed and an adjacent bed. .Iaddend..Iadd.48.The method of claim 37 including steps of: identifying resistivity dropsin said measured log; and using said resistivity drops as an indicatorof a boundary between said bed and an adjacent bed. .Iaddend..Iadd.49. Amethod of locating a substantially horizontal bed of interest in aformation and maintaining a drillstring therein during the drillingoperation, said drillstring including a measurement-while-drilling (MWD)resistivity sensor, comprising the steps of:generating a modeledresistivity log for a substantially horizontal bed using assumedresistivity values for a desired course of drilling from a preselecteddipping bed model, said modeled resistivity log being indicative of theresistivity taken along the substantially horizontal bed in a formation;drilling a directional well in said formation, a portion of saiddirectional well intended to be disposed in said substantiallyhorizontal bed; measuring resistivity in said directional well using theMWD resistivity sensor to provide a log of resistivity takensubstantially horizontally; and comparing said measured log to saidmodeled log to determine the location or position of said directionalwell with respect to said substantially horizontal bed..Iaddend..Iadd.50. The method of claim 49 including the steps of:adjusting the directional drilling operation so as to maintain saiddrillstring within said substantially horizontal bed during the drillingof said directional well in response to said comparing step..Iaddend..Iadd.51. A method of locating a substantially horizontal bedof interest in a formation and maintaining a drillstring therein duringthe drilling operation, said drillstring including ameasurement-while-drilling (MWD) resistivity sensor, comprising thesteps of:generating a group of dipping bed model logs for variousexpected downhole conditions; drilling a directional well in aformation, a portion of said directional well intended to be disposed ina substantially horizontal bed; measuring resistivity in saiddirectional well using the MWD resistivity sensor to provide a real timelog of resistivity taken substantially horizontally; comparing said realtime log to at least one of the model logs in said group of dipping bedmodel logs until one of said model logs substantially matches said realtime log defining a matched model log; and using input parameters fromsaid matched model log to determine the location or position of saiddirectional well with respect to said substantially horizontal bed..Iaddend..Iadd.52. The method of claim 51 including the step of:adjusting the directional drilling operation so as to maintain saiddrillstring within said substantially horizontal bed during the drillingof said directional well in response to said comparing step..Iaddend..Iadd.53. A method of locating a substantially horizontal bedof interest in a formation and maintaining a drillstring therein duringthe drilling operation, said drillstring including ameasurement-while-drilling (MWD) resistivity sensor, comprising thesteps of:drilling a directional well in a formation, a portion of saiddirectional well being disposed in a substantially horizontal bed;measuring resistivity in said directional well using the MWD resistivitysensor to provide a real time log of resistivity taken substantiallyhorizontally; generating a model resistivity log from a preselecteddipping bed model for a substantially horizontal bed using input valuesselected so that the model resistivity log will substantially match thereal time log defining a matched model log; and using input parametersfrom said matched model log to determine the location or position ofsaid directional well with respect to said substantially horizontal bed..Iaddend..Iadd.54. The method of claim 53 including the step of:adjusting the directional drilling operation so as to maintain saiddrillstring within said substantially horizontal bed during the drillingof said directional well in response to said comparing step..Iaddend..Iadd.55. For use in connection with an earth borehole drillingapparatus that includes a drilling rig, a drillstring operating fromsaid drilling rig for drilling an earth borehole, said drillstringincluding a bottom hole arrangement comprising a drill bit, a downholeresistivity measuring subsystem for measuring downhole formationresistivity near said bit by propagating electromagnetic energy intoearth formations near said bit, receiving electromagnetic energy thathas propagated through the formations and producing measurement signalsthat depend on the received signals; a method for directing the drillingof a well bore with respect to a geological bed boundary in said earthformations, comprising the steps of:producing from said measurementsignals a recording of downhole formation resistivity as a function ofborehole depth, said formation resistivity being in the form of a logdepicting phase difference and amplitude ratio; identifying separationsbetween said phase difference and amplitude ratio in said log; andimplementing a change in the drilling direction of said drill bit inresponse to said identification of any separations between said phasedifference and amplitude ratio. .Iaddend..Iadd.56. For use inconjunction with an earth borehole directional drilling apparatus thatincludes a drilling rig, a drillstring operating from said drilling rigfor drilling an earth borehole, said drillstring including a bottom holearrangement comprising a drill bit, a downhole resistivity measuringsubsystem for measuring downhole earth formation resistivity near saidbit by propagating electromagnetic energy into earth formations nearsaid bit, receiving electromagnetic energy that has propagated throughthe formations and producing measurement signals that depend on thereceived signals and downhole telemetry means for transmitting themeasurement signals; wherein said apparatus has initiated drilling awell bore within a first geological bed in said formations andapproximately adjacent a geological bed boundary between said first bedand a second geological bed in said formations; a method for maintainingdrilling of the well bore in said first bed, comprising the stepsof:deriving from said measurement signals, downhole formationresistivity, said formation resistivity being in the form of a logdepicting phase difference and amplitude ratio; monitoring saidresistivity, as a function of the borehole depth of the drill bitposition; identifying separations between said phase difference andamplitude ratio in said log; and implementing changes in the drillingdirection of said drill bit in response to the identification of anyseparations between said phase difference and amplitude ratio..Iaddend..Iadd.57. For use in conjunction with an earth boreholedrilling apparatus that includes a drilling rig, a drillstring operatingfrom said drilling rig for drilling an earth borehole, said drillstringincluding a bottom hole arrangement comprising a drill bit, a downholeresistivity measuring subsystem for measuring downhole formationresistivity near said bit, receiving electromagnetic energy that haspropagated through the formations and producing measurement signal thatdepend on the received signals;a method for directing the drilling of awell bore with respect to a geological bed boundary in said earthformations, comprising the steps of:producing from said measurementsignals a recording of downhole formation resistivity as a function ofborehole depth; identifying resistivity drops in said resistivityrecording; and implementing a change in the drilling direction of saiddrill bit in response to said identification of any resistivity drops..Iaddend..Iadd. . For use in conjunction with an earth boreholedirectional drilling apparatus that includes a drilling rig, adrillstring operating from said drilling rig for drilling an earthborehole, said drillstring including a bottom hole arrangementcomprising a drill bit, a downhole resistivity measuring subsystem formeasuring downhole earth formation resistivity near said bit bypropagating electromagnetic energy into earth formations near said bit,receiving electromagnetic energy that has propagated through theformations and producing measurement signals that depend on the receivedsignals, and downhole telemetry means for transmitting the measurementsignals uphole and uphole telemetry means for receiving the measurementsignals; wherein said apparatus has initiated drilling a well bore witha first geological bed in said formations and approximately adjacent ageological bed boundary between said first bed and a second geologicalbed in said formations; a method for maintaining drilling of the wellbore ins aid first bed, comprising the steps of:deriving from saidmeasurement signals, downhole formation resistivity; monitoring saidresistivity as a function of the borehole depth of the drill bitposition to detect any resistivity drops; and implementing changes inthe drilling direction of said drill bit in response to the detection ofany resistivity drops. .Iaddend..Iadd.59. A method of locating a bed ofinterest in a formation and maintaining a drillstring therein during thedrilling operation, said drillstring including ameasurement-while-drilling (MWD) sensor, comprising the steps of:(1)modeling at least one substantially horizontal bed to provide a modeledlog of a set of parameters indicative of the response taken as the MWDsensor traverses or moves within the bed, said modeling being based oninformation related to said substantially horizontal bed and the path ofthe MWD sensor; (2) drilling a directional well, a portion of saiddirectional well intended to be disposed in said substantiallyhorizontal bed; (3) measuring and recording MWD sensor output parametersindicative of the formation parameters in said directional well usingthe MWD sensor to provide a measured log of said output parameters; and(4) comparing the measured log to said modeled log to determine thelocation or position of said directional well with respect to saidsubstantially horizontal bed. .Iaddend..Iadd.60. The method of claim 59including the step of: adjusting the directional drilling operation soas to maintain said drillstring within said substantially horizontal bedduring the drilling of said directional well in response to saidcomparing step. .Iaddend..Iadd.61. The method of claim 59 wherein: saidinformation is selected from at least one of (a) data from an offsetwell which intersects with said horizontal bed, (b) seismic data, (c)assumptions of the horizontal bed, and (d) prior geological knowledge ofthe horizontal bed. .Iaddend.